The present invention relates generally to the field of disc-drive data storage systems and, in particular, to track search capability within such systems.
The term "disc-drive systems" (or "disk-drive systems") is directed to any system (e.g., optical, magnetic, etc.) that accesses data held on a rotating disc. Optical disc-drive systems include read-only compact discs (CD), digital versatile discs (DVD) and digital videodiscs (DVD), as well as their writable counterparts (e.g., CD-R, CD-RW, DVD-R and DVD-RAM). In such systems, information is read from and/or written to a disc by a transducing head or "pickup" supported adjacent the disc surface. Among the most common of these optical systems is CD-ROM.
CD-ROMs store data in a single, spiral track (analogous to a phonograph record) that circumnavigates the disc thousands of times (e.g., over 20,000) as it gradually moves away from the center of the disc. For ease of discussion, each rotation of this single, spiral track is referred to herein as a track. The architecture and operation of CD-ROM drive systems are well-known by those having ordinary skill in the art, and a description of such systems may be found in C. Sherman, CD-ROM Handbook, Intertext Publications McGraw-Hill, Inc. (1994), which is hereby incorporated by reference in its entirety for all purposes.
FIG. 1A shows a simplified block diagram of a typical optical disc-drive system used to reproduce data from a CD-ROM (i.e., a CD-ROM drive system). In optical disc-drive system 10, spindle motor 14 rotates CD-ROM disc 12; optical pickup unit ("OPU") 20 reads data stored on CD-ROM 12 using an optical pickup; feed motor (or "sled motor") 22 changes the radial position of OPU 20; microprocessor controller 40 and a collection of servo and control circuitry (e.g., focusing servo 26, tracking and feed motor servo 24) command disc-drive system 10 to perform the desired operations. CD-DSP 30 is a digital signal processor which descrambles the signal read from CD-ROM 12 by OPU 20 and provides, via CD-ROM controller 39, the digital output data via bus 44 to host computer 50. CD-ROM controller 39 is typically an ATAPI/IDE or SCSI based device, as is well known in the personal computer field.
As noted above, optical disc-drive system 10 reads the data stored on the CD-ROM through OPU 20. OPU 20 translates the optically stored data into electrical signals by shining a laser beam onto CD-ROM disc 12 and detecting the strength of the reflection using photodiodes. The reflected beam contains both data and tracking information. Referring again to system 10, servos 26 and 24 control OPU 20 to keep the laser beam in focus and on track. Finally, CD-DSP 30 demodulates the data signal read from disc 12 and performs the necessary error detection and correction to reproduce and output the stored information.
FIG. 1B illustrates an exemplary arrangement of photodiodes within an optical pickup 100 (disposed in OPU 20) which are used to detect the reflected beam from disc 12. At the center of the arrangement is a four-quadrant photodiode containing diode quadrants or photodiodes 102, 104, 106, and 108, which generate signals A, B, C and D, respectively. A difference in the output current from each quadrant indicates that the laser beam is out of focus on disc 12. A focus error signal is generated by subtracting the signal representing the sum of the currents from diodes 104 and 108 from the signal representing the sum of the currents from diodes 102 and 106. Focusing servo 26 uses the focus error signal (FE) to move an objective lens in OPU 20 in the direction of disc 12 to bring the beam into focus on disc 12. The sum of the output currents from all four quadrants represents the data signal itself. The data signal is coupled to data processing unit 30 where it is reshaped into its original form and delivered to host computer 50.
On either side of photodiodes 102-108 are photodiodes 110 and 112, which generate signals E and F, respectively. These signals are used to ensure the pickup stays correctly on track. The tracks on a CD-ROM disc are tightly spaced, where the track pitch is only 1.6 .mu.m. Eccentricities in the disc can cause radial swings up to 300 .mu.m. Thus, maintaining proper tracking of the incident beam is an essential part of the disc drive operation. One commonly used approach is the "three-beam" laser design, in which OPU 20 generates two side beams alongside a main beam. The side beams are used primarily for radial mistracking correction. Photodiodes 110 and 112 are disposed to receive the reflection from these side beams. The intensity of reflection from the sides beams should be equal if the main beam is correctly on track. The difference in the current output from photodiodes 110 and 112 (that is, signal E-F) forms a tracking error signal (TE). The. tracking error signal is used by servo 24 to command a tracking actuator in OPU 20 (not shown) via signal TRO in order to fine tune the radial position of OPU 20, keeping OPU 20 (or, more specifically, the optical pickup) on track. The operation of maintaining the pickup over a desired track is referred to as "track following."
Disc-drive system 10 navigates a CD-ROM disc 12 by searching for a desired track among some 20,000 tracks on the disc. Optical pickup unit 20, besides being moved to correct for tracking errors, is also moved radially across disc 12 to search for a desired track. During a track search, optical pickup 100 (FIG. 1B) as part of OPU 20 moves in a direction shown by arrow 114, which is perpendicular to track orientation arrow 116 (approximating track orientation on an optical disc). The operation of positioning the pickup over a destination or target track by radially moving the pickup across one or more tracks is referred to as a track "search" or "seek." System 10 determines the distance traveled and derives the velocity of track traversal by counting the number of tracks crossed over (i.e., "track crossings"). These crossings are detected by sinusoidal signals generated by photodiodes 102-108 and photodiodes 110, 112. As described below, the resulting signals ("RFRP" and "TE," respectively) oscillate in conjunction with track crossings during a track search.
A conventional track search method includes a coarse search operation combined with a fine search operation. In a coarse search operation, OPU 20 traverses the majority of the distance between the current position and the desired track. Disc-drive system 10 then reads the current track information and calculates the number of remaining tracks to be traversed. System 10 then executes a fine search operation (which may be over hundreds of tracks) where OPU 20 moves inward or outward the appropriate number of tracks remaining. In a conventional disc-drive system, because of inaccuracy in counting track crossings, the fine search operation may be repeated several times before the desired track is finally reached.
FIG. 2A illustrates well-known elements of system 10 used to process track-crossing signals. These elements, identified collectively as preamp/servo circuit 25, may be disposed partly in RF amplifier 19 and partly in tracking and feed motor servo 24 of system 10. Referring to FIG. 2A, the elements of circuit 25, for purposes of discussion, are collectively identified in two separate circuits: preamplifier circuit 118 and servo circuit 120. Inputs to circuit 118 include signals E and F from photodiodes 110 and 112, respectively (FIG. 1B), and signals A, B, C and D from diode quadrants or photodiodes 102, 104, 106 and 108, respectively. Signals E and F are input to difference preamplifier 352 producing signal E-F, which is filtered by low pass filter 124 to produce tracking error signals TE and TEO. Additionally, signals A, B, C and D are input to summing preamplifier 302 producing a data signal referred to as "RF".
When OPU 20 is positioned on a track center, RF is simply a data signal containing high frequency components. However, when OPU 20 is traversing the disc, the summed signal is modulated. The modulation, or envelope, of this summed signal is in quadrature (i.e., 90.degree. out of phase) with the tracking error signal (TE). Utilizing this quadrature relationship, track crossing counts can be qualified and direction of pickup motion can be accurately determined. RF and TE signals are further discussed in connection with FIG. 2B.
Referring again to FIG. 2A, track crossing circuit 122 receives signal E-F (output from preamp 352) for counting the number of track crossings during a rough search operation. This information is forwarded to full track counter 130 for further processing, as is known in the art. As noted above, low-pass filter 124 also receives signal E-F, and forwards a resulting "tracking error" signal (stripped of high frequency components) to comparator 358 (as signal TE) and tracking equalizer 132 (as signal TEO). As is known in the art, TEO may be used to control the radial positioning of OPU 20 to keen the main laser beam on track through the use of tracking equalizer 132 and feed motor equalizer 134. These equalizers generate tracking output ("TRO") and feed motor output ("FMO") signals, respectively, which are selectively applied through switches 154 and 150 under the control of microprocessor controller 40. Additionally, tracking error signal TE is forwarded to comparator 358 where it is compared with a Vref signal and thereby converted to square wave TX. Exemplary TE signal 226 and TX signal 228 are illustrated in FIG. 2B.
Referring again to FIG. 2A, signals A, B, C and D are supplied to summing preamplifier 302. The output of preamplifier 302 contains a high frequency carrier signal which in effect is the data signal. When optical pickup unit 20 is traversing a CD-ROM disc during a search operation, the summed signal is modulated; depicted in FIG. 2B as RF signal 230. The envelope of the modulation 232 approximates a low frequency sinusoidal waveform. When OPU 20 is at track centers, such as at lines 222 and 224, the amplitude of the envelope is the greatest. On the other hand, when OPU 20 is maximally off track boundaries (i.e. exactly between two adjacent tracks), the modulating signal is weakest.
As shown in FIG. 2A, preamplifier 302 is coupled to peak/bottom detector 126. This detector receives the RF signal from preamplifier 302 and processes this signal by repeatedly detecting peak and bottom values of the RF signal and dynamically determining a center value 235. In detector 126, peak detection has slow dynamics while bottom detection has fast dynamics. Utilizing these dynamics, the resulting signal, radio frequency ripple signal ("RFRP") 234 in FIG. 2B, gradually has its center value redefined at the start of a search to approximate the midpoint of the RFRP sinusoidal wave (see, signal portion 233 in FIG. 2B). As shown in FIG. 2B, signal 234 suffers from considerable noise. Returning to FIG. 2A, RFRP is input to comparator 308 where it is compared with a Vref signal and thereby converted to square wave RX. Exemplary RX signal 236 is illustrated in FIG. 2B.
In a conventional track search, pickup 100 crosses tracks of an optical disc by moving along the radius of the disc in the direction of arrow 114 (FIG. 1B). During this process, signals RFRP and TE result in sinewaves approximately 90.degree. out of phase from each other (i.e., quadrature in phase), as shown in FIG. 2B. To facilitate processing, RFRP is thresholded by comparator 308 (FIG. 2A) to produce square wave RX. Similarly, TE is thresholded by comparator 358 to produce square wave TX. The phase of RX in view of TX (i.e., approximately 90.degree. ahead or behind) indicates direction of a search and may be used to qualify counts, as discussed below.
The physical relationship of signal TE 226 with optical disc tracks is shown schematically in FIG. 2B. Referring to this figure, TE 226 crosses a level zero 250 (i.e., V.sub.ref) at disc track centers 222 and 224. As shown therein, the period "T" of TE 226 represents the crossing of one track width or pitch (e.g., 1.6 .mu.m). Each crossing of level zero by TE corresponds to a change in state by TX, which is referred to as a half-track pulse 450. Accordingly, starting at point 454 of TE 226 (and not counting the "half-track pulse" created at this point), the generation of two half-track pulses 450 occurring at points 456 and 458 will represent the traversal of a single track on a disc by pickup 100.
RX and TX signals are fed to difference counter 136 which relies upon the quadrature relationship between TX and RX to ensure an accurate count. Difference counter 136 is loaded by controller 40 (FIG. 1A) with target and direction information (e.g., +200 tracks). The value loaded is then counted down using TX-based signals. The TX signals themselves are "qualified" by the RX signals. Specifically, counter 136 may be programmed to monitor for cycles where RX first rises, then TX rises, then RX falls and finally TX falls; i.e., a quadrature relationship where RX precedes TX. If this cycle is satisfied, the count provided by the TX signals is "qualified" and can be relied upon as accurate. However, if this cycle is not detected, the quadrature relationship has been lost and the data can no longer be relied upon.
Returning to FIG. 2A, the output of counter 136 (i.e., half-track pulses, as is well known in the art) is fed to velocity servo 140 to create FMO or TRO control signals. The application of these signals through switches 150 and 154, to feed motor 22 and OPU 20, respectively, is controlled by microprocessor controller 40 as is well known.
A number of deficiencies exist with the prior-art tracking servo of FIG. 2A. Referring to FIG. 2B, RFRP signal 234 generated by detector 126 of FIG. 2A is noisy and prone to inaccuracy. Upon initiating a search, conventional searching start-up noise combined with the inherent noise of RFRP and the process of redefining a center value for RFRP (signal portion 233) results in inaccurate RX signals, as represented by signal portion 237. These inaccurate RX signals, in turn, cause counter 136 to miscount resulting in track search inaccuracy; i.e., the wrong number of track crossings are "counted" during a search creating the need for a subsequent search. Additionally, filter 124 introduces a phase shift in signal TE, which increases with the frequency of this signal. Detector 126 also introduces a phase shift in signal RFRP, but this shift is a different value from that in TE. As the frequency of these signals increases, the difference in phase shifts between these signals also increases until, ultimately, the quadrature relationship is lost causing difference counter 136 to fail. Finally, peak/bottom detector 126 typically fails at signal frequencies exceeding about 40 kHz because it is unable to respond fast enough to signal oscillations at such frequencies. Accordingly, detector 126 creates a frequency ceiling which inherently limits the usefulness of this circuit in high-speed fine searches.
An additional problem facing prior-art systems, such as tracking servo 24, is the gradual degradation of analog signals TE and RFRP during the course of operation. Referring to FIG. 2C, signal 202 represents a sinusoidal signal oscillating with track crossings and derived from the summation of signals from photodiodes 102-108 (i.e., A+B+C+D). Similarly, signal 204 represents a sinusoidal signal oscillating with track crossings and derived from the difference of signals from photodiodes 110 and 112 (i.e., E-F). Like RFRP and TE described above, signals 202 and 204 function as track crossing signals representing the movement of an optical pickup across one or more tracks during a track search.
During a track search, signals A+B+C+D and E-F ideally result in sinusoidal waves 90.degree. out of phase from each other, oscillating about a reference voltage Vref as shown in FIG. 2C. Vref is typically ground or an offset ground in a single power supply system.
Ideally, signals 202 and 204 should have a 50--50 duty cycle, which contributes to more accurate detection of track crossings. Signals 202 and 204 of FIG. 2A are shown in this ideal state; i.e., they are above level zero 220 for about one half of their period and below this level for the other half (thereby representing a symmetric or 50--50 duty cycle).
However, in practice, this 50--50 duty cycle may not initially be achieved due to an unwanted DC bias on the subject analog signal (i.e., A+B+C+D and/or E-F) which creates an offset from symmetric operation. As is well known, this unwanted bias may be substantially nullified by applying a correction voltage or bias to the subject signal.
Referring to FIG. 3, a sinewave 402 (representing E-F in this example) is subject to an offset 406 from V.sub.ref. In accordance with conventional methods, this offset is determined by peak detecting the top and bottom of wave 402. Since signals from photodiodes and preamplifiers are noisy, average top and bottom peak values are calculated over a relatively large number of periods (i.e., "T" of FIG. 4) of the subject wave. Typically, the peaks of thirty-two or sixty-four full sinewaves (generated over thirty-two or sixty-four periods, respectively) are measured to obtain the necessary values for calculating the offset. Once calculated, a correction bias 404 is adjusted to produce a new correction bias 404', which compensates for the undesired offset 406. The corrected wave 402' achieves an approximate 50--50 duty cycle about V.sub.ref.
The foregoing conventional method requires considerable time (i.e., 32 or 64 sinewave periods) to collect the required samples for averaging peak values. As such, this method has an inherent latency that is problematic when performing a track search operation since undesired offsets of A+B+C+D and/or E-F typically undergo rapid change during such searches.
Moreover, the foregoing conventional method is typically applied only once at spin-up calibration (i.e., during power up of a disc-drive system). However, optical pickup signals such as RFRP and TE have been observed to gradually deteriorate during the course of a track search when the pickup is in motion. As such, an initially-applied correction bias may be gradually rendered ineffective over the life of a single search.
Additionally, the conventional method is highly sensitive to sinewaves A+B+C+D and E-F being clipped or similarly distorted since an accurate offset can only be determined from accurate peak values.
Further, the conventional method produces a correction signal that is applied in its entirety at one time. If the offset is large, a comparable correction signal can introduce large transients into the servo loop used for tracking operations which may cause tracking reliability problems.
The foregoing discussion highlights inadequacies in current systems to produce consistently reliable analog signals utilized in monitoring track crossings. Accurate counting of track crossings enhances the positioning control of the optical pickup in OPU 20. Errors in track counts cause OPU 20 to be mispositioned, prolonging the track search time. Furthermore, reliable track crossing counts are needed to optimize the traversal velocity of OPU 20. During the track search operation, the ability to precisely control the acceleration and deceleration of OPU 20 can improve seek time significantly.
Thus, it would be desirable to generate more reliable analog signals utilized in monitoring track crossings to improve the accuracy of track searches performed in optical disc-drive systems.